Magnetographic printers are well known, and are preferred in information processing systems that require printing machines capable of printing highly legible characters at extremely high speeds (several thousand or even ten thousand lines per minute).
Such a printer is described in French Patent No. 2,522,857, filed on Mar. 3, 1982. This type of magnetographic printing machine (also known as a non-impact magnetic printer) includes a magnetic recording carrier comprising a magnetic drum, in turn formed by a magnetic shunt comprising its central portion, on which a magnetic recording layer is deposited. The machine also includes a set of magnetic recording heads, placed one beside the other parallel to the axis of rotation of the drum. These magnetic heads make it possible to create magnetized domains or points on the surface of the recording layer of the drum, which is driven by a roller bearing with uniform rotation. Magnetized zones comprising a set of magnetized domains are thus formed; the shape of the zones correspond to that of the characters to be printed. These magnetized domains are then coated with particles of a powdered magnetic pigment by means of a developer device. This pigment for instance comprises magnetic particles coated with a resin. The resin tends to melt when heated and is affixed to the printing paper to which it is applied. The magnetic pigment adheres to the sets of magnetic domains as defined above, forming a deposit of particles on the drum surface. These particles are then transferred to a sheet of paper pressed against the drum by a transfer roller. The particles that remain on the drum are then lifted off by an erasing device.
In view of the above explanation, it can be appreciated that in a magnetographic printer, the magnetic drum is an essential device. In fact, the printing quality of the characters and the homogeneity of printing over the entire surface of the printing paper depends on the magnetic properties of this drum and on their homogeneity over its entire surface.
Hence it is particularly important to be able to assure that in the course of the various successive manufacturing operations for producing the drum, the drum will have magnetic properties over its entire surface that conform to reference norms arrived at in advance, for example by experimentation. These norms define the standard magnetic characteristics that the drum must have in order for the printing quality to be correct (that is, the curve of primary magnetization track by track, the resulting permeability, and the coercive field, all of them being over the entire surface of the drum).
The terms "determining", "defining", and "characterizing" may be used interchangeably when referring to ascertaining the magnetic characteristics of a drum, layer or film.
It is important to be able to monitor the magnetic characteristics of a drum as soon as the operations of manufacturing the drum are completed, and to do so before the drum is coated with a layer for mechanical protection against shock and corrosion (this layer is for instance of chromium).
Under current circumstances, the procedure is as follows: At the same time as the magnetic recording layer is deposited on the magnetic shunt comprising the central portion of the drum, a representative sample of the drum is made up. This sample comprises a specimen made of brass, on which a magnetic material is deposited that is strictly identical (in terms of both the constituent material and the thickness) to that comprising the recording layer of the drum. The various magnetic characteristics of this sample are measured, for instance, with a commercially available magnetometer or a conventional flux meter. Verification is done as to whether these properties do conform to the above-defined reference norms. If so, then the drum is sent to a chromium-plating station, where it is coated with its protective chromium layer.
It will be understood that the characterization of this sample cannot precisely reflect the magnetic properties of the drum in its entirely. It often happens, moreover, that on returning from the chromium-plating station the magnetic drum is found not to have the required magnetic properties over its entire surface for assuring correct printing quality. It is then rejected, which is very expensive.